Multilayered plating processes are known and disclosed as means for inhibiting corrosion mechanisms such as the galvanic effect, which arises where different metals having different electromotive potentials contact each other directly or through a conductive medium. The differing electromotive potentials of the metals create an electrical potential between the two metals causes corrosion to occur upon the surface metals.
Other factors contribute to the corrosion of metal layers, such as the chemical nature of the metal, the purity of the metal, the physical and mechanical condition of the metal, and the environment to which the metal is exposed.
A review of the prior art reveals many different methods of electrodepositing, electroplating multiple layers of different metals as a means for inhibiting corrosion. For instance, in U.S. Pat. Nos. 4,601,958 and 4,666,796 to Levine it is disclosed that metal parts for the sealing of semiconductor packages comprising an iron based alloy layer is electroplated with a first layer of nickel, then electroplated with a first layer of gold, then electroplated with a second nickel layer, and then electroplated with a second gold layer. In U.S. Pat. No. 3,708,405 to Kamata, it is disclosed that a copper alloy wire is electroplated first with a layer of nickel, second with a layer of gold, third with another layer of nickel, and fourth with another layer of gold.
U.S. Pat. No. 4,835,067 to Levine discloses an electroplating process suitable for use as sealing lids or cover elements for semiconductor packages in which substrate layers that are nickel containing iron alloys are electroplated with a base layer of a metal with an electromotive potential high with respect to that of the substrate, over which an intermediate layer is electroplated, which intermediate layer has an electromotive potential which is low with respect to the base layer, and over that a cover layer is plated which has an electromotive potential similar to that of the base layer. It is further disclosed that suitable combinations of the electroplated layers are gold-nickel-gold combinations.
Other multilayered electrodepositing methods utilizating gold are known in the art. U.S. Pat. No. 3,963,455 to Nobel discloses the electrodepositing of a tungsten-cobalt alloy or tungsten nickel alloy layer between the base metal and the gold electrodeposit layer in order to prevent a barrier to diffusion.
A review of the above-referenced and other prior art reveals that while many electroplating processes and their applications have been developed, no such applications adequately solve the unique problems encountered in the jewelry industry.
Because of the prohibitive cost of gold, jewelry manufactured today is rarely composed of solid gold. Rather, most jewelry is composed of base metals such as brass, which is then plated with a gold finish. Brass is the favored base material because it may be cast while in its molten form into many different shapes. A layer of bright nickel is ordinarily employed in combination with the brass. Nickel is a preferred component because it creates a shiny metallic appearance. However, because of the electromotive potential that results when brass and nickel are in contact with each other, a jewelry product of this composition is prone to corrosion that tarnishes its appearance, an aesthetically unpleasing condition that ruins the inherent beauty of the article.
In jewelry products, corrosive effects are due to the galvanic effect, the environment in which the jewelry is worn, and the physical and mechanical conditioning of the metal. Environmental factors which can cause corrosion are contact with water, sweat and other moisture, all of which serve as a conducting medium for the flow of electrons. For this reason there is a greater likelihood of corrosion where the jewelry comes in contact with moisture.
The manufacture of the jewelry itself may also act as a corrosion promoting mechanism. Jewelry is machined during manufacture, stressing the metal and creating fissures where corrosion can occur. Of course, such stressing is not limited to the manufacturing process, as the care exhibited by the jewelry owner is an important factor. Scratches, nicks, etc. occurring when the jewelry is worn create fissures which promote corrosive effects.
The conventional process which has been employed in the art is to electrodeposit a layer of bright nickel upon the brass substrate, and then to electroplate a layer of gold upon the outer surface. However, these products have a limited ability to resist corrosion and will eventually break down, as depicted in FIG. 1, which is a reprint of a FIG. 6-7 of Faust, "Corrosion and Protective Coating", Metals Engineering Institute, p. 6-10, 1977.
A corrosion pit can form where a noble (i.e. less negative) metal M.sub.1 on the outside surface is plated over a metal M.sub.2 which is less noble (i.e.--more negative) and is upon base metal M.sub.3. As can be seen, this coating method does nothing to prohibit galvanic action, as electrons flow from the substrate layer up to the intermediate layer, and then up to the surface layer.
Sometimes a thick layer of gold or another intermediate layer is applied over the nickel to assure longer wear. Under these situations, the product often experiences corrosion originating from interactions between base metal, the undercoat layers, and covering layer of gold.
An added factor is that the product is exposed to an assortment of plating solution additives which are, used to improve the brightness, leveling, and/or luster of product. These additives tend to accelerate delimination, corrosion, or failure of the outer and inner coatings.